The typical tractor includes a continuous drive belt rotatably mounted on the tractor body and driven by a drive sprocket which is rotated by the drive shaft of the printer mechanism In operation, most tractors have projecting pins on the outer surface of the moving drive belt which engage in perforations in the paper web and advance it as the lines of print are formed thereon.
The tractors are typically used in pairs disposed along the side edges of the perforated web. Spaced from and parallel to the drive shaft is a support shaft, and the tractors are slidable on both shafts to adjust the spacing therebetween. Because sheet material webs of different widths may be used, it is desirable that the tractors permit clamping and unclamping to be performed with relative ease.
Most clamping mechanisms are disposed to one side of the body of the tractor, i.e., offset to one side of the centerline of travel of the belt. When the shafts are not perfectly straight, this may result in canting from the desired orientation or produce torque upon rotation of the drive shaft as the center-to-center spacing varies due to aberrations in one or both shafts. Moreover, the act of clamping may cause the body to move slightly on the support and drive shafts from the desired position of alignment of the centerline of the drive belt in the centerline of the perforations in the web. This could occur if the clamping force exerts a force on the support shaft which is axially disposed from the centerline, and this can even induce minor movement of the tractor on the support shaft.
Additionally, when the clamping action is axially displaced from the centerline of the belt, a moment arm exists between the point of clamping and the center line or drive axis of the belt. This may produce "creep" of the tractor on the support shaft due to the torque produced by the drive belt.
Accordingly, it is an object of the present invention to provide a tractor with an improved clamping mechanism located within the body of the tractor.
It is also an object to provide such a tractor in which the components may be readily fabricated and assembled.
Another object is to provide such a tractor which may be readily clamped and unclamped and which is rugged and long lived in operation.